Huixuan Zhang

Thermal-mechanical fatigue behavior of a cast K417 nickel-based superalloy

The thermal-mechanical fatigue (TMF) behavior of cast K417 nickel-based superalloy was investigated under in-phase (IP) and out-of-phase (OP) loading in the temperature range from 400 to 850°C. The results revealed that the tendency to cyclic hardening under thermal-mechanical and isothermal fatigue was higher than that under static tensile testing at 850°C. Isothermal fatigue (IF) was observed to cause higher cyclic flow stress than TMF. At a corresponding strain amplitude, the thermal-mechanical fatigue life was lower than that of isothermal fatigue, and the TMF life of out-of-phase cycling was higher than that of in-phase cycling. Scanning electron microscopic observations of fracture surfaces and longitudinal sections revealed intergranular fracture under in-phase TMF which led to the decrease in fatigue life.

Structure evolution and mechanism of polyacrylonitrile and related copolymers during the stabilization

The structure evolution and thermal behaviors of polyacrylonitrile homopolymer (PAN) and terpolymer [P(AN–MMA–IA)] containing about 4.19 wt% methyl methacrylate (MMA), 0.98 wt% itaconic acid (IA) and 94.83 wt% acrylonitrile (AN) during heat treatments were studied by Fourier transform infrared (FTIR), differential scanning calorimetry and thermogravimetry. It was found that the presence of IA comonomer in PAN promoted the cyclization reactions significantly though initiating the cyclization reactions at lower temperature and proceeding at a faster rate than these of PAN homopolymer. Moreover, it was found that the dehydrogenation was based on the formation of cyclized structures. From the results of FTIR analysis, we found the MMA comonomer blocked the cyclization reactions to some extent. Extent of cyclization reactions at different temperatures was calculated through the data of FTIR. The sequence of reactions happened during the stabilization was oxidationxa0>xa0cyclizationxa0>xa0dehydrogenation. The higher rate constant k implied that the ionic mechanism actually showed a kinetic advantage at low temperature over the free radical mechanism, even though the MMA comonomer inhibited cyclization reactions to some extent.

Effects of tungsten fiber on failure mode of Zr-based bulk metallic glassy composite

The authors systematically investigated the effects of tungsten fiber on failure mode as well as deformation and fracture mechanisms in tungsten fiber-reinforced Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk metallic glassy composite under uniaxial compression at room and high temperatures. At room temperature, the failure mode of the composite changes from shear fracture to longitudinal splitting failure with increasing fiber volume fraction. Similar to the observations in monolithic metallic glasses, the shear fracture angle of the composite is approximately equal to 39∼40 deg, indicating that the Mohr-Coulomb criterion is suitable to give the critical shear fracture condition of the composite. When the compression tests were performed below the glass transition temperature of Zr41.25Ti13.75Ni10Cu12.5Be22.5 metallic glassTg, the deformation behavior of the composite strongly depends on the strain rates and the test temperature, which is quite similar to the deformation behavior of monolithic metallic glasses in the supercooled liquid region. The corresponding failure mode of the composite changes from shear or splitting fracture to bending failure with decreasing strain rate or increasing test temperature. The failure modes at the temperature nearTg are mainly controlled by the metallic glass matrix due to the decrease in its viscosity at high temperature. Based on these multiple failure modes, the effects of test temperature and tungsten fiber volume fraction on deformation and fracture mechanisms are summarized.

Large-scale and narrow dispersed latex formation in batch emulsion polymerization of styrene in methanol–water solution

This work is an extension of previous research results reported by our team (Colloid Polym Sci 291:2385–2389, 2013), where monodisperse, large-scale, and high-solid-content latexes of poly(n-butyl acrylate) were obtained with the particle coagulation method induced by the electrolyte. However, large-scale polystyrene latex particle is difficult to synthesize with this approach; moreover, demulsification phenomena easily take place especially in high solid content. In this article, a new approach to prepare large-scale polystyrene latex particle was proposed. Methanol was added to aqueous phases to decrease the interfacial tension between the polymer particle surface and continual phases, further decreasing interfacial free energy. Consequently, the surfactant molecules would loosely pack on the polymer particle surface, which is favored by particle coagulation. Experimental investigations showed that the final polystyrene particle scale only reaches to 93.5xa0nm when the methanol/water ratio is equal to 0:100, but the particle size attains 270xa0nm when the methanol/water ratio is equal to 30:70. These results indicated that polystyrene particle coagulation can be induced by methanol by varying the surfactant molecule adsorption on the particle surface. This investigation also provided a new simple approach to prepare large-scale, stable latex particles.

Cyclic deformation and fatigue crack propagation of a Zr-based bulk amorphous metal

The behavior of the cyclic deformation and fatigue crack propagation Of Zr41.25Ti13.75Ni10Cu12.5Be22.5 (in at.%) bulk amorphous metals is investigated. Both fully amorphous and partially crystallized metals are characterized in order to understand the fatigue behavior of these different microstructures. Under fully reversed load control, both amorphous metals show stable cyclic strain response until final fracture. The partially crystallized metal shows an inferior fatigue resistance in comparison with that of the fully amorphous metal. The absence of definite crystalline microstructure in the amorphous metal and a relatively small area of stable crack propagation on the fracture surface are main reasons for the stable strain response under cyclic deformation. Similar to the ductile crystalline alloys, the fatigue crack propagation of both amorphous metals obeys Paris power-law relationship at intermediate growth rates. Both fully amorphous and partially crystallized metals show a similar law of fatigue crack propagation in that their values of threshold and fatigue exponents m are comparable. The mechanisms of fatigue crack propagation in the region of near-threshold and intermediate crack growth are similar to that of ductile crystalline alloys. The crystalline phases in the partially crystallized metals can retard the crack or act as sites of crack initiation when they are oriented at a suitable direction for that of crack propagation

Synthesis of monodisperse, large scale and high solid content latexes of poly(n-butyl acrylate) by a one-step batch emulsion polymerization

Seeded emulsion polymerization and agglomerating method were well-known techniques for the production of polymeric latexes with large particle size and high solid content. Obtaining latexes with monodisperse and particle size above 300xa0nm scale, however, was time-consuming and difficult by means of these methods. Here, stable, monodisperse latexes with the controlled particle diameter (55–650xa0nm) and high solid content (60xa0wt%) were synthesized via one-step in batch emulsion polymerization. Experimental investigations show that the particle size increased with decreasing emulsifier concentrations and increasing monomer/water ratios or electrolyte concentrations. The latex particle coagulation was considered as the dominant particle formation and growth method, which could be proved by the evolutions of particle number as well as dimension against conversion. Latex particle coagulation occurred if the particle surface covered ratio dropped between the critical surface covered ratio (θcritu2009=u20090.59) and the lowermost surface covered ratio (θlowu2009=u20090.38). In addition,θcrit and θlow were increased with electrolyte concentrations.

Thermal, rheological, and mechanical properties of polylactide/poly(diethylene glycol adipate)

Polylactide (PLA) was plasticized with a new biodegradable macromolecular plasticizer-poly(diethylene glycol adipate) (PDEGA). The crystallization behavior, miscibility, rheological behavior, mechanical properties and phase behavior of PLA/PDEGA blends were investigated. The PDEGA lowered the glass transition temperature and the cold crystallization temperature. With an increase of PDEGA content, the break strain and impact strength increased significantly. The high break strain of 480xa0% and the high impact strength of 30xa0kJ/m2 were obtained for 70/30 PLA/PDEGA blend. PDEGA was uniformly dispersed in the PLA matrix. The results indicated that PDEGA had a good plasticizing effect on PLA.

Hydrophilicity of polymer effects on controlled particle coagulation in batch emulsion polymerization

The poly(methyl methacrylate-co-styrene) was prepared by batch emulsion polymerization to clarify the effect of characteristics of polymer on particle coagulation. Experimental results showed that the size of final latex particle increased with increasing methyl methacrylate in initial recipe, ranged from 84 to 193xa0nm, which was attributed to the particle coagulation. With the methyl methacrylate increased, the hydrophilicity of polymeric particle improved, thus led to the surfactant molecules packed loosely on the polymer surface, further, enhanced particle coagulation occurred. On the contrary, the surfactant molecules adsorbed on tightly the polymeric particle surface (methyl methacrylate content low) surface led to the electrostatic repulsion energy of polymer particle improved, and polymer particle stability was also improved. Thus, combined with the results previously reported by us (Colloid Polym Sci 291: 2385–2398, 2013 and Colloid Polym Sci 292: 519-525, 2014), the particle coagulation depended not only on the aqueous phase such as electrolyte concentration and methanol content, but also on the nature of polymer such as hydrophilicity.

Crosslinking network structure effects on particle coagulation in the emulsion polymerization of styrene in methanol solution

This work is an extension of previous research results reported by our team (Colloid Polym Sci 292:519–525; 1347–1353), where how to control particle size distribution by adjusting particle coagulation has been investigated. There is limitation in previous studies, that is, the particles taking part in coagulation only possess a linear structure. However, the crosslinking network structure is necessary to many fields, such as rubber and plastic modifier. Thus, it is very significant to investigate the effect of the crosslinking network structure on particle coagulation for understanding the nature of particle coagulation and controlling particle size distribution. In this manuscript, dihydrodicyclopentadienyl acrylate (DCPA) is chosen as a crosslinker to copolymerize with styrene to promote particle structure shift from the linear to the crosslinking network structure and to investigate further the coagulation behavior of the particles with the crosslinking network structure. Experimental results are explained by the collision frequency of particles and stability dependence of the particle structure.

Preparation of monodisperse, sub-micrometer polymer particles by one-step emulsion polymerization under particle coagulation

The particle coagulation technology in emulsion polymerization is a novel and facile approach to prepare large-scale, narrowly dispersed latex particles. However, the formation of narrowly dispersed latex particles under particle coagulation at a high zeta potential is surprising. To elucidate this observation, a detailed investigation on the relationship between particle coagulation and particle size distribution was carried out. Unlike the conventional emulsion polymerization, a rapid decrease in the particle number and an increase in the particle size were clearly observed during the polymerization. The results confirm the occurrence of particle coagulation. The width of zeta potential and particle size distribution also decreased with particle coagulation, resulting in large-size, narrowly dispersed latex particles. These phenomena were explained by the competitive growth mechanism.